1. Field of the Invention
The present invention relates to adjustable support devices, and more particularly to adjustable support devices for typists and other swivel chairs of the type having a seat part which can be raised or lowered relative to a base part.
2. Description of the Prior Art
It is conventional in typists chairs to provide a seat part which is mounted above a base part by a supporting column which includes a screw-jack comprising a nut and a screw. The component parts of the screw-jack are secured respectively to the seat part and base part so that by rotation of the seat part the height of the seat can be raised or lowered. The disadvantage of this construction is that swivelling movement of the seat in use will also produce an unwanted increase or decrease in the seat height.
There has been proposed in British Patent Specification No. 647,183, a supporting column in which the nut and the screw are automatically locked together when the chair is occupied in order to prevent unwanted seat height adjustment from occuring. This effect is achieved by using a split nut which is clamped onto the screw under the loading acting on the column when the seat is occupied. The use of a split nut with means for clamping the nut onto the screw complicates the construction and is liable to fail in use.